1. Field of the Invention
The present invention relates to a mass spectrometer and method of mass spectrometry.
2. Discussion of the Prior Art
Time of Flight mass analysers are well known wherein packets of ions are ejected by an electrode such as a pusher electrode into a field free drift region with essentially the same kinetic energy. In the drift region ions with different mass to charge ratios travel with different velocities and therefore arrive at an ion detector disposed at the exit of the drift region at different times. Measurement of the ion transit time therefore determines the mass to charge ratio of that particular ion.
One of the most commonly employed ion detectors in Time of Flight mass spectrometers is a single ion counting detector in which an ion impacting a detecting surface produces a pulse of electrons by means of, for example, an electron multiplier. The pulse of electrons is typically amplified by an amplifier and a resultant electrical signal is produced. The electrical signal produced by the amplifier is used to determine the transit time of the ion which struck the detector by means of a Time to Digital Converter (“TDC”) which is started once a packet of ions is first orthogonally accelerated into the drift region. The ion detector and associated circuitry is therefore able to detect a single ion impacting onto the detector.
However, such ion detectors exhibit a certain dead-time following an ion impact during which time the detector cannot respond to another ion impact. A typical detector dead time may be of the order of 1–5 ns. If during acquisition of a mass spectrum ions arrive during the detector dead-time then they will consequently fail to be detected, and this will have a distorting effect on the resultant mass spectra. At high ion currents multiple ion arrivals cause counts to be missed resulting in mass spectral peaks with lower intensity than expected and inaccurate mass assignment.
It is known to use dead time correction software to correct for distortions in mass spectra. Statistical dead time correction can successfully correct intensity and centroid measurement to within acceptable levels up to a signal corresponding to a well defined average number of ions per pushout event i.e. a well defined average number of ions per energisation of the pusher electrode. However, software correction techniques are only able to provide a limited degree of correction. Even after the application of dead time correction techniques, ion signals resulting in more than one ion arrival on average per pushout event at a given mass to charge value will result in saturation of the ion detector and will thus result in a non-linear response and inaccurate mass determination.
This problem is particularly accentuated with gas chromatography and similar mass spectrometry applications because of the narrow chromatographic peaks which are typically presented to the mass spectrometer which may be, for example, only a few seconds wide at their base.
It is therefore desired to provide an improved mass spectrometer and method of mass spectrometry.